1. Field of the Invention
The present invention relates to a circular polarized wave antenna and a method for designing the same, which is used for a radio communication system such as satellite communication.
2. Description of the Related Art
A position detecting system such as so-called “GPS” (Global Positioning System), satellite radio broadcasting, and the like are radio communication systems using circular polarized wave. A transceiver (transmitting and receiving apparatus) corresponding to such a radio communication is generally provided with a circular polarized antenna.
As a representative example of the circular polarized wave antenna, a single feed microstrip antenna disclosed in the “Small size plane antenna” by Misao Haneishi et al, 1996, Institute of Electronics, Information and Communication Engineers, pages 142 to 164, has been known. In the single feed microstrip antenna, two modes spatially orthogonal to each other are generated by providing a degeneracy separating element, to transmit and/or receive the circular polarized wave. Concerning electrical dimensions of the single feed microstrip antenna as shown in this document, a length of one side is about a ½ wavelength at a frequency to be used. For example, the length of the single feed microstrip antenna is about 95 mm at a frequency of 1575 MHz which is used for the GPS. Since such dimensions are too large for an antenna for a mobile transmitting and receiving apparatus, the size of the single feed microstrip antenna is reduced by using a high dielectric material such as ceramic, and broadly put to practical use.
As explained above, a small-sized circular polarized wave antenna is required in the mobile transmitting and receiving apparatus. So as to solve this problem, various small-sized circular polarized wave antennas have been studied. For example, Japanese Patent Laid-Open No. 05-152830 and Japanese Patent Laid-Open No. 08-051312 disclose techniques of forming the small-sized circular polarized wave antennas.
In a microstrip antenna disclosed by Japanese patent Laid-Open No. 05-152830, notches are provided at both terminals of a conductor along directions of two straight lines that are orthogonal to each other with an angle of ±45° with respect to two oscillation mode directions having different phases and orthogonal to each other, so that any one of the straight lines orthogonal to each other with an angle of ±45° with respect to the oscillation mode directions coincides with a deviated direction of a power feed point, thereby lowering an oscillation frequency. According to this structure, the antenna size can be reduced compared with that of the conventional antenna used at the same frequency.
In a circular polarized wave loop antenna disclosed by Japanese Patent Laid-Open No. 08-051312, a liner antenna element comprising a loop as a basic element is used for reducing the size of the antenna.
According to a conventional technique for providing a microstrip antenna disclosed by Japanese patent Laid-Open No. 05-152830, it is possible to reduce the antenna size by keeping the two modes spatially orthogonal to each other that are necessary for transmitting and receiving the circular polarized wave. However, there is a difficulty in adjusting impedance. Although the technique for adjusting reactance is disclosed, repetition of trial and error is required for adjusting resistance.
According to a conventional technique for providing a circular polarized wave loop antenna disclosed by Japanese Patent Laid-Open No. 08-051312, it is possible to transmit and receive the circular polarized wave by adjusting an aspect ratio of a rectangular loop element and a distance between the element and the ground. However, there is also a difficulty in adjusting impedance. Although there are described a technique for adjusting the impedance by loading a reactance on a part of a radiating element and a technique for adjusting an impedance by connecting a plurality of radiating elements in parallel, miniaturization of the antenna is reversely hindered as a result.
Therefore, a small-sized circular polarized wave antenna with high possibility of design and a method for designing the same have been desired.